What Is the Definition of a "mammal"?

Mammals, including humans, belong to the Mammalian class, which is the highest order of vertebrates. Mammals have a backbone, grow hair, produce milk for the nourishment of their young via mammary glands and are warm-blooded, or endothermic.

As endothermic creatures, mammals regulate their body temperature, which remains constant regardless of their surroundings. Mammals also use lungs to breathe air and most give birth to live offspring, with the exception of the monotremes. The platypus is a member of this group of mammals that lay eggs for reproduction. A mammal's skeleton is distinguishable because the lower jaw bone connects directly to the skull. Mammals, however, vary greatly in terms of size and appearance